Process of preparing toluic acids



July 5, 1955 L. K. CHENEY 2,712,549

PROCESS OF PREPARING TOLUIC ACIDS Filed Oct. 23, 1953 /0 REACTIO 5 CONDEN5ER\ VESSEL $333 2 SEPARATOR CENTRIFUGE 2a 24 45 L. Z 6 19 /-CENTR|FUGE INVENTOR 1 ma A. CHENEY ATTORNEYS United States Patent Qficc 2,712,549 Patented July 5, 1955 PaocEss or rnnrannso roLtuc ACIDS Leroy K. Cheney, Arcadia, Califi, assignor to Richfield Oil Corporation, Los Angeles, Calif a corporation of Delaware Application Gctober 23, 1955, Serial No. 388,037

3 Claims. (Cl. 266-524) My invention relates to improvements in the manufacture of toluic acids by oxidizing the corresponding xylenes.

It is known to prepare toluic acids by oxidizing xylenes in liquid phase using as a catalyst a cobalt, manganese or cerium salt which is soluble in the reaction mixture. The oxidation is carried out under elevated conditions of temperature and pressure in the presence of air or other oxygen-rich gas used as the oxidizing medium. The individual xylene isomers or mixtures thereof can be used as the starting material.

The oxidation of xylenes is a highly exothermic reaction. in the commercial practice of the oxidation reaction, a rapid rate of oxidation and high throughputs of reactants are desirable in order to operate at a reasonable production rate for a given plant size. This can be accomplished only if the large amounts of heat produced in the oxidation can be removed readily and quickly. External or internal heat exchangers are often used in commercial practice for this type of exothermic process. However, I have found that when the usual type of heat exchangers are used in the xylene oxidation system they become fouled very rapidly, lose their elliciency, and are very costly to operate.

1 have observed that in the oxidation of xylenes, varying amounts of phthalic acids are produced in the oxidation mixture in addition to the production of toluic acids and intermediate oxidation products. Phthalic acids are unique in this system and are distinguished from toluic acids in the reaction mixture by their substantial insolubility. As a result, the phthalic acids precipitate from the reaction mixture, particularly on heat exchange surfaces, cutting down their etficiency and it difficult or costly to maintain proper reaction conditions.

My invention is an improvement in the general process just described for the manufacture of toluic acids in which phthalic acids are also produced. My invention contemplates a series of processing steps which make possible adequate heat control and hence proper reaction temperature over an extended period of operating time and avoids the difliculty normally encountered as a result of phthalic acid production.

I do this by carrying out the xylene oxidation in a conventional reaction vessel. In addition to withdrawing efiiuent product containing toluic acids and phthalic acids from this vessel for further purification, i also withdraw a stream which is flashed into a tank maintained at a pressure not exceeding about atmospheric ressure. From this flash tank there is removed overhead a vapor stream which comprises water vapor and xylenes. This vapor stream is condensed to produce a water layer and a xylene layer. The Water layer is discarded and the xylene layer is again introduced into the reaction vessel. Bottoms from the flash tank are also introduced into the reaction vessel.

For a more complete understanding of the improved method which I have devised, reference is made to the accompanying drawing which sets forth in diagrammatic form an arrangement of processing equipment in which a preferred form of my method can be carried out. Fig. 1 will be ex lained in connection with typical operating conditions which can be employed in operating the equipment for toluic acid production.

Through line 1 there is introduced at the rate of pounds per hour and at 38 C. a petroleum xylene fraction having an API gravity of 35.3 and having a boiling range closely approximating that of xylenes. This fraction contains 82 per cent by weight of aromatics and 2.2 per cent by weight of olefins, the remainder being paraffins and naphthenes. The aromatics are mixed xylene isomers and about 10 per cent by weight of ethyl benzene, based on aromatics. xylene fraction has admixed with it cobaltous toluate in the amount of 0.4 per cent by weight, based upon the weight of the petrolum xylene fraction. The pctroleum xylene fraction introduced into line 1, together with the cobaltous toluate catalyst, passes through line 2 into cylindrical reaction vessel 3, which has a length of about four feet and a diameter of about two feet. Reaction vessel 3 is also provided with line 4 into which is introduced air in the amount of 500 cubic feet per hour, measured at F. and one atmosphere, with line 5 for the removal of off-gas, with line 6 for the removal of liquid efi'iuent product and with line 7 for the removal of a stream of reaction mixture for flashing, condensing and return to reaction vessel 3, as will be more fully explained.

In reaction vessel 3 there is maintained a liquid level of reaction mixture having a temperature of C. and a pressure of p. s. i. g. Line 7 is provided with valve 8 which controls the flow of the reaction mixture to flash tank 9. In flash tank 9 there is maintained a liquid level at 2 p. s. i. a. and 77 C. The reaction mixture is transferred through line 7 at the rate of 620 pounds per hour. Vapors including water vapor and xylenes pass overhead from flash tank 9 through line 16 and are condensed in condenser 11. The materials condensed in condenser 11 are transferred to separator 12 through line 13 at 38 C. Two liquid phases are present in separator 12, the upper one being essentially xylenes and the lower being essentially water. The water layer is withdrawn through line 14 and discarded and the xylenes are transferred back to the reaction vessel through lines 15 and 16 by means of pump 17. The dash tank bottoms are returned to the reaction vessel through lines 18 and 16 by means of pump 19. Liquid is pumped through line 15 at the rate of 225 pounds per hour. Liquid is pumped through line 18 at the rate of 390 pounds per hour. Line 20 is connected to a conventional source of vacuum (not shown).

The toluic acids and phthalic acids produced as a result of the xylene oxidation pass through line 6 and thence into enclosed continuous centrifuge 29 which serves to separate the phthalic acids. Air is present while the centrifuging operation is being carried out, in order to maintain the cobalt catalyst in soluble form. The liquid remaining after the centrifuging has been performed passes through line 21 into hold tank 22, wherein a liquid level is maintained. A gas which has little or no oxygen content, namely, a mixture of 99 er cent of nitrogen and 1 per cent of oxygen, is introduced into hold tank 22 by means of line 23 and is removed by means of line 24. As a result, the cobalt catalyst is converted into an insoluble form. By means of line 25 the product mixture, containing precipitated cobalt catalyst, is introduced into enclosed continuous centrifuge 26, wherein the insolubilized cobalt catalyst is separated. Further details concerning the insolubilization of the cobalt catalyst are set forth in Chester M. Himel Patent No. 2,680,757. The liquid eflluent from This petroleum centrifuge 26 passes through' line 27 and can be handled as describedinLester P. Berriman andChester M. Himel application, Serial No. 321,488, filed November 19, 1952. Liquid flows through line 27 at the rate of approximately 40; pounds per hour andlis composed of 7 about 25 per cent by weight of toluic acids, 15 per cent by weight of other oxidation products and 60 per cent by weight of xylenes.

As those skilled in the art will realize, various modifications can be made in the specific procedures just described to provide other embodiments which fall within the scope of my invention. In general, the operating temperature used in carrying out the oxidation will be from about 130 to about 190 C. and the operating pressure used will be from about 50 to about 500 p. s. i. g. (preferably 100 p. s. i. g. to 250 p. s. i. g.). Details concerning the oxidation of xylenes to form toluic acids are set forth in Chester M. Hirnel application, Serial No. 296,718, filed July 1, 1952.

I claim:

1. In the preparation of toluic acids by oxidizing the corresponding xylenes, the steps of introducing xylenes and an oxygen-rich gas, into a reaction vessel wherein the xylene is oxidized in a liquid phase maintained under elevated conditions of temperature and pressure and containing as a catalyst a salt which is soluble in the reaction mixture and which is selected from the group consisting of cobalt salts, manganese salts and cerium salts, the severity of the oxidation conditions being such that phthalic acid is formed in addition to toluic acid, withdrawing from the reaction vessel a stream of the reaction mixture present therein, flashing such stream into a flash tank maintained at a pressure not exceeding about atmospheric pressure, removing a vapor stream comprising water vapor and xylene overhead from the flash tank, condensing the water vapor and xylene and separating them, returning liquid Xylene and bottoms from the flash tank to the reaction vessel, and withdrawing oft-gas and a liquid product stream from the reaction vessel.

2. A process according to claim 1 in which the liquid product stream from the reaction vessel is filtered to remove phthalic acid, is transferred to a hold tank in which there is maintained an atmosphere of a substantially oxygen-free gas whereby the catalyst is converted to an insoluble form, and in which the liquid product stream is thereafter again filtered to remove the insolubilized catalyst.

3. A process according to claim 1 in which the catalyst is a cobalt salt.

No references cited. 

1. IN THE PREPARATION OF TOLUIC ACIDS BY OXIDIZING THE CORRESPONDING XYLENES, THE STEPS OF INTRODUCING XYLENES AND AN OXYGEN-RICH GAS INTO A REACTION VESSEL WHEREIN THE XYLENE IS OXIDIZED IN A LIQUID PHASE MAINTAINED UNDER ELEVATED CONDITIONS OF TEMPERATURE AND PRESSURE AND CONTAINING AS A CATALYST A SALT WHICH IS SOLUBLE IN THE REACTION MIXTURE AND WHICH IS SELECTED FROM THE GROUP CONSISTING OF COBALT SALTS, MANGANESE SALTS AND CERIUM SALTS, THE SEVERITY OF THE OXIDATION CONDITIONS BEING SUCH THAT PHTHALIC ACID IS FORMED IN ADDITION TO TOLUIC ACID, WITHDRAWING FROM THE REACTION VESSEL A STREAM OF THE REACTION MIXTURE PRESENT THEREIN, FLASHING SUCH STREAM INTO A FLASH TANK MAINTAINED AT A PRESSURE NOT EXCEEDING ABOUT ATMOSPHERIC PRESSURE, REMOVING A VAPOR STREAM COMPRISING WATER VAPOR AND XYLENE OVERHEAD FROM THE FLASH TANK, CONDENSING THE WATER VAPOR AND XYLENE AND SEPARATING THEM, RETURNING LIQUID XYLENE AND BOTTOMS FROM THE FLASH TANK TO THE REACTION VESSEL, AND WITHDRAWING OFF-GAS AND A LIQUID PRODUCT STREAM FROM THE REACTION VESSEL. 